Conventionally, digital audio is encoded into a file with a pulse-code modulation (PCM)-based format, such as a WAV audio file, with a sample rate of 44.1 kHz. This sample rate results in critically-sampled digital audio that is only sufficient to capture the 20 to 20 kHz audible range for human hearing. When the digital audio is played back by an electronic device, the audio data is input to a digital-to-analog converter (DAC), which in turn drives an amplifier that powers speakers. These DACs often operate at a significantly faster rate than the sample rate of the digital audio, that is a much faster rate than 44.1 kHz. The DACs thus employ oversampling of the input digital audio data to operate on symbols, or units of information, at a much higher sample rate (e.g., in the MHz) than the originally sampled data (e.g., 44.1 kHz). The oversampling places audio distortion that may be caused by aliased images far outside the audible range. Oversampling DACs use digital low pass filters to remove the aliased images before digital-to-analog conversion. However, the low input sample rate (relative to the human limit of hearing) requires that the filters used are high order filters with narrow transition bands. These high-order filters can allow audio up to 20 kHz (the human hearing limit) through but attenuate anything above approximately 24 kHz. These high-order filters provide a “brick wall” frequency response that results in significant spreading of impulse response energy in time. Users may be able to discern the audio distortions introduced by these high order filters and may attribute unnatural sounding and harsh music reproduction to these steep anti-aliasing filters. An illustration of such a system is shown in FIG. 1A.
FIG. 1A is a block diagram illustrating an example playback path with an oversampling digital-to-analog converter (DAC) according to the prior art. A playback path 120 may begin with an audio source 122 that provides data to zero-stuffing block 124 for interpolation. The zero-stuffed audio data is filtered at digital low pass filter (LPF) 126, rate converted at block 128, and filtered again in digital low pass filter 130. The output of digital LPF 130 may be provided to modulator 132, DAC 134, and amplifier 136 for output to speaker 138. The operation of zero-stuffing block 124 and filters 126 and 130 may result in pre- and post-ringing and large group delay in the time domain that may result in audio distortion and/or create up-sampling images in the frequency domain that may result in audio distortion. One solution to avoid the audio distortion created by zero-stuffing block 124 is to use a non-oversampling system that converts critically sampled audio directly from digital to analog, as shown in FIG. 1B.
FIG. 1B is a block diagram illustrating an example playback path for digital audio according to the prior art. A playback path 100 may begin with an audio source 102 that provides digital data to a Nyquist-rate DAC 104 (or “NOS DAC”). The analog output of the Nyquist-rate DAC 104 may be provided to an analog low pass filter 106 and then to an amplifier 108 for output to a speaker 110. Although some audio distortion is avoided through the use of the playback path 100 of FIG. 1B, other audio distortion is added by the playback path 100. This distortion is reduced or removed through the use of the analog low pass filter (LPF) 106 and the inherent filtering effects of the amplifier 108 and speaker 110.
Further, other drawbacks exist with such a system. For example, such a system relies on the DAC 104, amplifier 108, speaker 110, and even the human ear to remove the up-sampling images created when the input data is upsampled. This system reliance leads to variations in perceived sound quality based on factors that cannot be controlled at design time, such as varying hearing capabilities of the listener. Another drawback is that addressing the above issues with additional filtering tends to make custom designs expensive, bulky, and power hungry through added electrical components. For example, a Nyquist digital-to-analog converter (NOS DAC), such as in FIG. 1B, may be one component that may be used to provide the DAC functionality and provide reduction or elimination of up-sampling images. However, NOS DACs are too large, bulky, and power hungry for mobile devices or many other electronic devices. Further, NOS DACs are relatively expensive for devices, and particularly for consumer-level devices.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for audio components employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.